Three-dimensional carbon structures

ABSTRACT

The method of the present disclosure is directed towards the formation of a three-dimensional carbon structure and includes the steps of adding a radical initiator to an amount of carbon starting material, forming a mixture, placing the mixture in a mold, maintaining the mixture and the mold at an elevated temperature for a period of time to form a thermally cross-linked molded mixture and removing the thermally cross-linked molded mixture from the mold. The disclosure also includes a three-dimensional carbon structure, with that structure including a thermally cross-linked carbon base material in a predetermined formation.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of a co-pending applicationhaving U.S. Ser. No. 14/390,914, filed on Oct. 6, 2014, which is a 371of International application having Serial No. PCT/US2013/035190, filedon Apr. 4, 2013, which claims the benefit of priority from U.S.Provisional Application No. 61/620,643, filed Apr. 5, 2012, the entirecontents of all of which are incorporated herein by reference.

BACKGROUND OF THE DISCLOSURE

Over the last decade, three-dimensional carbon scaffolds have beenfabricated using various techniques such as chemical vapor deposition,substrate patterning and capillary-induced self-assembly. However, theseapproaches present a practical challenge to develop further carbondevices; either due to scalability issues, or high operational cost.

Three-dimensional microscopic scaffolds using carbon nanotubes havepreviously been assembled via techniques such as pattern transfer,stereo-lithography, focused ion beam lithography and chemical vapordeposition; collectively referred to as “pre-patterned” or “bottom-up”approaches. Various “top-down” approaches such as capillary-inducedself-assembly and nanotube-polymer hybrids offer the potential ofcheaper, and potentially scalable methods for the fabrication ofthree-dimensional scaffolds with carbon nanotubes. Using thesestrategies, three-dimensional structures of carbon nanotubes have beensynthesized. However, the suitability of these top-down and bottom-upapproaches as a versatile method to fabricate three-dimensional allcarbon scaffolds with various carbon nanomaterials, such as fullerenesand graphene, has not been demonstrated.

Embodiments of the present application provide methods and structuresthat address the above and other issues.

SUMMARY OF THE DISCLOSURE

The present disclosure is directed to three-dimensional carbonnanostructures and methods of making the same. The method of forming thethree-dimensional carbon structure includes the steps of adding aradical initiator to an amount of carbon starting material, forming amixture, placing the mixture in a mold, maintaining the mixture and themold at an elevated temperature for a period of time to form a thermallycross-linked molded mixture and removing the thermally cross-linkedmolded mixture from the mold. The method also includes the addition of ahalogenated methane to the mixture prior to the slurry being placed inthe mold. The method also includes the halogenated methane beingchloroform. The method also includes the carbon starting material beingselected from the group consisting of multiwalled carbon nanotubes(MWCNT), single-walled carbon nanotubes (SWCNT), fullerenes andgraphene. The method also includes the thermally cross-linked moldedmixture being greater than or equal to 1 millimeter in at least onedimension. The method also includes the thermally cross-linked moldedmixture being less than 1 millimeter in all dimensions. The method alsoincludes a radical initiator selected from the group consisting of aperoxide, benzoyl peroxide, a compound with a peroxide functional group(ROOR′), methyl ethyl ketone peroxide, Di-tert-butyl peroxide, benzoylperoxide, acetone peroxide, bisacylphosphine oxide (BAPO),aluminophosphate (MAPO), tert-Amylperoxy-2-ethyl (TAPO) orazobisisobutyronitrile (AIBN) and combinations thereof.

The method also includes the thermally cross-linked molded mixturehaving a pore size in the range of about 125 nanometers to about 200micrometers. The method also includes the thermally cross-linked moldedmixture structure having porosity in the range of about 20% to about85%. The method also includes the ratio of the carbon starting materialto the amount of radical initiator being in the range of about 1:0.5 toabout 1:4.

The disclosure is also directed towards a three-dimensional carbonstructure, the structure including a thermally cross-linked carbon basematerial in a predetermined formation. The structure also includesporosity in the range of about 20% to about 85%. The structure alsoincludes the predetermined formation of the structure being greater thanor equal to 1 millimeter in at least one dimension. The structure alsoincludes the predetermined formation of the structure being less than 1millimeter in all dimensions. The structure also includes a pore size inthe range of about 125 nanometers to about 200 micrometers.Three-dimensional carbon structures of the present disclosure can beseen in FIGS. 1 and 2.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be better understood by reference to thefollowing drawings of which:

FIG. 1 is a photographic representation of a three-dimensional carbonstructure fabricated from multiwalled carbon nanotubes (MWCNT);

FIG. 2 is a photographic representation of three-dimensional carbonstructures fabricated from single-wall carbon nanotubes, fullerenes andgraphenes;

FIG. 3 is a graphical representation of the Raman spectra of unmodifiedMWCNTs and the Raman spectra of a three-dimensional carbon structure ofthe present disclosure;

FIG. 4A-4D are photographic representations at different levels ofmagnification of three-dimensional carbon structures of the presentdisclosure;

FIGS. 5A-5D are graphical representations of the pores of athree-dimensional carbon structure of the present disclosure; and

FIG. 6A-6C are a graphical representation of porosity ofthree-dimensional carbon structures of the present disclosure withvarying MWCNT to benzoyl peroxide ratios

DETAILED DESCRIPTION

The disclosure includes a method of forming a three-dimensional carbonstructure. The method for forming a three-dimensional carbon structurehas several steps, the first being the addition of a radical initiatorto an amount of carbon starting material to form a mixture of the two.The radical initiator can be any suitable peroxide, including but notlimited to benzoyl peroxide, any compound containing a peroxidefunctional group (ROOR′) including but not limited to, methyl ethylketone peroxide, Di-tert-butyl peroxide, benzoyl peroxide, and acetoneperoxide and the radical initiator can be bisacylphosphine oxide (BAPO),aluminophosphate (MAPO), tert-Amylperoxy-2-ethyl (TAPO) orazobisisobutyronitrile (AIBN). The carbon starting material can be anysuitable carbon starting material, including but not limited tomultiwalled carbon nanotubes (MWCNT), single-walled carbon nanotubes(SWCNT), fullerenes, grapheme and any carbon network that containing Pibonds such as, but not limited to dicarbon bonds. The ratio of additionof each component, carbon material to radical initiator, can be anysuitable ratio, including in the range of about 1:0.5 to about 1:4. Thismethod can form three-dimensional, free standing, all-carbon scaffoldsprepared via radical initiated thermal cross-linking of a suitablestarting material, such as MWCNTs that will possess robust structuralintegrity and stability.

Upon mixture of the radical initiator and carbon starting material, athermal cross-linking process begins via radical initiation by theradical initiator. Radical initiators, including peroxides, are used asan initiator in a free radical polymerization reaction with the suitablecarbon starting material. The radical initiator thermally decomposes toyield phenyl free radicals, and CO₂ gas, and is used for covalentfunctionalization of carbon starting materials. The phenyl radicalformed via decomposition of the radical initiator attacks the pi bondcarbon network on the suitable carbon starting material structure;thereby forming active centers, which serve as inter-nanotube andinter-material cross-linking sites.

Following the mixture of the radical initiator and the suitable carbonstarting material, there is an optional step of adding a solvent,chloromethane or halogenated methane, such as chloroform, to themixture. The optional additive acts to dissolve the radical initiatorand aid in a more uniform dispersion of the radical initiator.

After the mixture of the radical initiator and the suitable carbonstarting material, or after the addition of a halogenated methanethereto, the mixture is placed into a prefabricated mold. The mold canbe in any suitable shape based on the shape of the desired end resultstructure. The mixture inside the mold and the mold itself are thenmaintained at an elevated temperature for a period of time. This periodof time can range anywhere from about 3 hours to about 72 hours, or fromabout 12 hours to about 40 hours or from about 20 hours to about 30hours. The elevated temperature can be any temperature suitable to allowcross-linking of the suitable carbon starting material. This elevatedtemperature can range anywhere from about 20° C. to about 100° C., orfrom about 40° C. to about 80° C., or from about 50° C. to about 70° C.,more specifically about any of the following temperatures 51° C., 52°C., 53° C., 54° C., 55° C., 56° C., 57° C., 58° C., 59° C., 60° C., 61°C., 62° C., 63° C., 64° C., 65° C., 66° C., 67° C., 68° C. or 69° C.Also, the mixture of radical initiator and suitable carbon startingmaterial can he photo cross linked instead of thermal cross-linked byexposure to ultraviolet (UV) light at varying wavelengths or other formsof ionizing radiation, such as x-ray and gamma. ray wavelengths.

After the period of time at the elevated temperature passes, a thermallycross-linked molded structure remains within the mold, which is thenremoved from the mold. The thermally cross-linked molded carbonstructure can be any size suitable for the specific application. In someinstances a relatively large structure is desired, one having a size ofgreater than or equal to 1 millimeter in at least one dimension. Inother instances, a relatively smaller structure is desired, one having asize less than 1 millimeter in all dimensions.

An optional step after removal of the molded carbon structure is anannealing step, which subjects the carbon structure to a temperature ofabout 150° C. for about 20 minutes. This temperature and time can alsobe any suitable temperature and time, including but riot limited toabout 100° C. to about 200° C. and from about 10 minutes to about 30minutes.

The thermally cross-linked molded carbon structure possesses nano-,micro- and macro-scale- interconnected pores. The porosity and pore sizeof the carbon structures can be controlled by varying the amount ofradical initiator added earlier in the method. Based on the amount ofadded radical initiator, the thermally cross-linked molded structurewill have a pore diameter in the range of less than 125 nanometers toabout 325 micrometers. The thermally cross-linked molded carbonstructure can also have a pore size in the range of about 250 nanometersto about 150 micrometers, or about 500 nanometers to about 100micrometers, or about 1 micrometer. to about 50 micrometers. Also, basedon the amount of added radical initiator, the thermally cross-linkedmolded structure will have a porosity in the range of about 20% to about95%. The thermally cross-linked molded carbon structure can also have aporosity at about any of the following percentages, 21%, 22% , 23%, 24%,25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%,39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%,53%, 54%, 55%, 56%, 57%, 58%, 59%, 60%, 61%, 62%, 63%, 64%, 65%, 66%,67%, 68%, 69%, 70%, 71%, 72%, %, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%,82%, 83%, 84% 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93% or 94%.

The disclosure also includes a three-dimensional carbon structurecomprising a thermally cross-linked carbon base material in apredetermined formation. The thermally cross-linked carbon base materialis created as described herein and can be formed into any shapedepending on desired use. The possible uses of the formedthree-dimensional carbon structure are copious, including the use of thestructures in clean technologies such as in energy storage, as acomponent of fuel cells, as high performance catalysts, supercapacitors, or as a component of photovoltaic cells, in thetelecommunication industry, such as components for absorbingelectromagnetic signals to send information between devices, in fieldemission devices, in smart sensors or other electronic devices, in thehealthcare industry as scaffolds for tissue engineering and variousimplants, membranes for filtration and as drug delivery vehicles.Although the above list includes many possible uses, the universe ofpossible uses for the described three-dimensional structure is muchgreater.

Depending on the desired use, the three-dimensional carbon structure canhave a. relatively large formation, such as at least one dimension ofthe structure being greater than or equal to 1 millimeter in at leastone dimension. For other desired uses, the three-dimensional carbonstructure can have a relatively small formation, such as that alldimensions are less than 1 millimeter.

The porosity of the three-dimensional carbon structure can be controlledby varying the amount of radical initiator utilized during its creationand can be tailored towards the desired use of the three-dimensionalcarbon structure. Based on the desired use, the three-dimensionalstructure can have a porosity in the range of about 20% to about 95% andhave a pore diameter in the range of less than 125 nanometers to about325 micrometers.

The carbon structure can also have a pore size in the range of about 250nanometers to about 150 micrometers, or about 500 nanometers to about100 micrometers, or about 1 micrometer to about 50 micrometers. Also,the three dimensional structure can have a porosity at about any of thefollowing percentages, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%,31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%,45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53, 54%, 55%, 56%, 57%, 58%,59%, 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%,73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83% or 84%.

The three-dimensional carbon structure includes favorable mechanicalproperties, such as hardness and elastic modulus. Based on the ratio ofthe carbon starting material to the amount of radical initiator, theelastic modulus of the carbon structure is from about 16 MPa to about 84MPa while the hardness of the carbon structure is from about 0.7 MPa toabout 4.9 MPa.

The following examples are provided to further illustrate the methodsand structures of the present disclosure and demonstrate some advantagesthat arise therefrom. It is not intended that the present disclosure belimited to the specific examples disclosed.

Example 1

In the following non-limiting example, multiwalled carbon nanotubes(MWCNT) (Sigma Aldrich, Cat No. 659258), single walled carbon nanotubes(SWCNT) (Sigma Aldrich, Cat No. 519308), fullerenes (Sigma Aldrich, CatNo. 483036), benzoyl peroxide (Luperox®, Sigma Aldrich, Cat No. 179981)and chloroform (Fisher Scientific, Cat No. BPC297) were used aspurchased. MWCNT, SWCNT and fullerenes were mixed with benzoyl peroxideat different weight ratios (1:0.5, 1:1, 1:2, 1:3 and 1:4) and 1 ml CHCl₃was added to the mixture. The mixture was poured in custom machinedTeflon molds (length=1.2 mm, diameter=0.5 mm) and incubated at 60° C.for 24 hours. Post incubation, the molds were disassembled andcross-linked three-dimensional carbon structures were obtained.

After incubation, Raman analysis was performed using a WITec alpha300RMicro-Imaging Raman Spectrometerusing a 532 nm Nd-YAG excitation laser.Point spectra were recorded between 50 -3750 cm⁻¹ at room temperature.

Micro-Computed Tomography (CT) analysis was performed to quantify thethree-dimensional porosity of MWCNT specimens. A Scanco MedicalmicroCT-40 was used at 45 kV and 177 μA current. All scans wereperformed in air by placing the sample in 12.3 mm field of view tubesand averaging the data 3 times. The threshold values were optimized toaccurately represent he raw images of the structures. Volumereconstruction and analysis of five cubes (900 μm³ volume) per samplewas performed using gauss sigma: 0.3, gauss support:1_, lower threshold:30 and upper threshold: 1000; by a vertebrae quantification script. Theregions of analysis were selected in the center of the structure toeliminate the edge artifacts. The porosity of the structures wascalculated as:

Porosity %=100−total volume % of objects in VOI

Scanning electron microscopy (SEM) was performed using JOEL 7600FAnalytical high resolution SEM, Structures were placed on a conductive,double sided, carbon adhesive tab (PELCO, Ted Pella) and imaged at 1 and5 kV accelerating voltages using a secondary electron imaging (SEE)detector. Transmission electron microscopy (TEM) was performed using FEIBioTwinG² TEM at Stony Brook University. -The samples were imaged at 80Vusing 300 mesh size, holey lacey carbon grids (Ted Pella, Inc.).

Image processing toolbox in MATLAB was used to quantify the porosityvalues of the structures. SEM images at various magnifications werecropped to remove the legend and the scale bar and were subjected toimage processing steps such as edge detection, thresholding, medianfiltration followed by quantification of region properties usingregionprops. Porosity was calculated using n=5 images as the ratio ofthe total area of voids to the total area of the image, using thefollowing formula:

Porosity(%)=(ΣArea of voids/area of the image)*100

In order to eliminate errors and have an accurate estimation of porosityfrom the SEM images, multiple images (n=5) of different magnificationswere analyzed. Statistical analysis was performed using a student's ttest and one-way anova followed by Tukey Kramer post hoc analysis. A 95%confidence interval (p<0.05) was used for all statistical analysis.

Liquid extrusion porosimetry (LEP) was performed on purified MWCNTscaffolds using the PMI liquid extrusion porosimeter at Porous MaterialsInc., Ithaca, N.Y. The CNT scaffolds were placed on a membrane and thesample chamber was filled with Gatwick® (wetting liquid, surface tension≈0, propene, 1,1,2,3,3,3-hexafluoro, oxidized, polymerized) whichpenetrates into the pores of the sample. An inert gas under pressure wasapplied to extrude the liquid from the pores of the MWCNT scaffold. Thevolume and weight of the extruded liquid was measured, and porosity andmedian pore diameter were calculated as described previously.

Example 2

In the following example, MWCNTs and benzoyl peroxide were mixed in theratio of 1:2 according to the described methods to form twothree-dimensional carbon structures of different dimensions. As can beseen in FIG. 1, the three-dimensional carbon structures formed of MWCNTsare robust, free-standing structures, and structurally stable. Althoughthe examples herein are described as being formed from MWCNTs,three-dimensional carbon structures can also be formed of SWCNTs,fullerenes such as C₆₀, and graphene, as seen in FIG. 2.

Results of Raman analysis of the three-dimensional carbon structures areshown in FIG. 3. Raman spectra of un-reacted MWCNTs (lower data line ofFIG. 3) and three-dimensional carbon structures (upper data line of FIG.3) are shown. The characteristic D band (1355 cm⁻¹), G band (1580 cm⁻¹)and G′ band (2694 cm⁻¹) were observed for un-reacted MWCNTs. Thethree-dimensional carbon structures showed peaks at 1000 cm⁻¹, 1230 cm⁻¹and 1775 cm⁻¹ in addition to the characteristic MWCNT spectra. The Gband in the Raman spectra has been attribute to the intrinsic vibrationof sp² bonded carbon atoms, whereas D band corresponds to the defectsinduced in the nanotube structure due to disruption of the sp² (C═C)bonds. I_(D)/I_(G) ratio for un-reacted MWCNTs was 0.07 which increasedto 0.85 for the three-dimensional carbon structures indicating thedisruption of the sp² bonded C═C domains.

Additional minor peaks at 1000 cm⁻¹, 1230 cm⁻¹ and 1775 cm⁻¹ wereobserved in the three-dimensional carbon structures, which can beattributed to the breathing mode (C—C stretching) of benzene ring, C—Obond stretching (vibration of the peroxide chain) and C═O bondstretching (aryl carbonate functional group), respectively. Theintensities of these peaks were relatively minor compared to the D and Gbands, and repeatedly observed in the Raman spectra. This data maysuggest that benzoyloxyl and phenyl radicals react with the MWCNTinitiating the cross-linking reaction to form the three-dimensionalcarbon structures.

In the case of the three-dimensional carbon structures, an increase inthe intensity of the D band was observed corresponding to the disruptionof the sp² carbon network due to the reaction of the benzoyl peroxidewith the MWCNTs.

Example 3

In the following example, MWCNTs and benzoyl peroxide were mixed inratios of 1:1 and 1:2 according to the described methods to form acarbon structure.

The mechanical properties of MWCNT scaffolds were determined usingnanoindentation (Triboindenter; Hysitron, Minneapolis, Minn.) with aBerkovich indenter tip. MWCNT scaffolds were attached to metal disksusing cyanocryolate and mounted into the indenter. The points ofindentation were selected at a distance no less than 100 μm away fromeach other. Samples were indented 7 times to determine elastic modulus(Er) and material hardness (H). The tip area function was calibratedfrom indentation analysis on fused quartz, and drift rates in the systemwere measured prior to each indentation using standard indentationtesting procedures. First, a preload of 3 μN was applied to the systemfollowed by a constant loading rate (10 μN/s). Then a hold segment at afixed system load was applied, followed by a constant unloading rate toretract the tip (−10 μ/s), then another hold segment was imposed (3 μN).The sample was indented with peak loads ranging from ≈15 μN to 100 μN.The elastic response was calculated from the 20% to 90% portion of theunloading curve using methods previously described.

Table 1 summarizes values of elastic modulus (Er) and hardness (H)measured by 7 indents (at least 100 μm distance between each indent). Erand H values of MWCNT scaffold (1:1) were 38.45±14.42 MPa and 1.82±0.54MPa, respectively. MWCNT scaffold (1:2) exhibited Er of 45.72±18.78 MPaand H of 3.47±1.73 MPa, higher than 1:1 MWCNT:BP scaffold. These elasticmodulus values for MWCNT scaffolds are significantly higher than thevalues measured for various polymeric, graphene and CNT based foams. Therelatively high values of elastic modulus and hardness of MWCNTscaffolds further indicates the formation of nanoscale, covalentcrosslinks between MWCNTs.

TABLE 1 Mechanical properties of MWCNT scaffolds determined bynanoindentation. MWCNT:BP 1:1 MWCNT:BP 1:2 Indent # Er (MPa) H (MPa) Er(MPa) H (MPa) 1 39.44 1.89 55.62 2.07 2 35.01 1.74 39.87 4.77 3 33.841.96 28.72 0.73 4 31.77 1.97 38.94 4.83 5 16.12 1.01 84.08 4.72 6 53.42.77 34.59 2.26 7 59.59 1.4 38.25 4.91 Mean ± 38.45 ± 14.42 1.82 ± 0.5445.72 ± 18.78 3.47 ± 1.73 SD

Example 4

In the following example, MWCNTs and benzoyl peroxide were mixed in theratio of 1:4 according to the described methods. Scanning electronmicroscopy (SEM) was also performed on the three-dimensional carbonstructures to characterize their structure, and confirm thecross-linking of the nanotubes. Some images captured from the SEM areshown in FIG. 4A-4D. FIG. 4A and 4B show representative low-resolutionSEM images of the formed three-dimensional carbon structures. Thecross-sections show interconnected MWCNT networks that form themacroscopic three-dimensional structures. The high resolution SEM inFIG. 4C and 4D also display the cross-linking between individual MWCNTs,and the formation of junctions (arrows, FIG. 4D). Unlike polymer chainsthat coil together tightly with no inter-chain space or airpockets, thecross-linked MWCNT network of the three-dimensional carbon structures isquite porous. The pores are irregular shaped and well-connected.

The porosity and pore size of the three-dimensional cross-linkedstructures was evaluated by Micro Computed Tomography (microCT) and SEMimage analysis. MicroCT was performed to assess the micro-porosity inthe three-dimensional cross-linked structure specimen. FIG. 5A displaysa three-dimensional reconstructed microCT image of a section of thethree-dimensional cross-linked structure specimen in air, FIGS. 5B, 5C,and 5D show the top, middle and bottom section of the three-dimensionalimage displayed in FIG. 5A, and indicating the presence of pores (whitevoids). These observations were consistent by going through allindividual cross-sections of the microCT reconstructed images withoutphysical sectioning. The analysis of the microCT slices determined thepore sizes to be between 100-300 μm. The pores were interconnected, anddistributed throughout the structure.

Example 5

In the following example, MWCNTs and benzoyl peroxide were mixed invarying ratios between 1:0.5 and 1:4 according to the described methods.

The microporosity of the three-dimensional cross-linked are shown inFIG. 6A-6C, arranged by ratio of MWCNTs to benzyl peroxide. The porositywas calculated as previously discussed in Example 1. As can be seen, theporosity can be manipulated by varying the amount of cross-linkingagent, benzoyl peroxide in this example. It should be noted that sincethe three-dimensional cross-linked structures were fabricated bycross-linking MWCNTs, the microCT system possessing a resolution of 6 μmwas not capable of detecting the presence of nano-sized pores.

As shown in FIG. 6A, the porosity is shown as it was determined bymicroCT data, as discussed above. Data represented in FIG. 6A is shownbelow in Table 2:

TABLE 2 Porosity of MWCNT scaffolds calculated from microCT analysis.MWCNT:BP Porosity (%) ratio by microCT   1:0.5 84.67 ± 1.70 1:1 79.26 ±1.77 1:2 70.29 ± 2.34 1:3 68.80 ± 5.72 1:4 21.31 ± 1.52

To further quantify the nano-porosity, image processing was performed ona series of SEM images, and the porosity was calculated as previouslydiscussed in Example 1. The porosity calculated by this methodcorresponds to the surface porosity and has been used to estimate theporosity values for sandstones and tissue engineering polymericscaffolds. The nano-porosity from this analysis for thethree-dimensional cross-linked structures was between about 65% andabout 85% and the pore sizes were between 125 nm-0.75 μm.

SEM image analysis of three-dimensional cross-linked structures alsoshowed that their nano-porosity could be manipulated by varying theamount of cross-linking agent, in this example benzoyl peroxide. FIG. 6Bdisplays the plot of porosity of various three-dimensional cross-linkedstructures (calculated from the SEM image analysis) fabricated by mixingMWCNTs with benzoyl peroxide at different weight ratios (1:0.5, 1:1,1:2, 1:3 and 1:4). The data represented in FIG. 6B is shown below inTable 3:

TABLE 3 Porosity of MWCNT scaffolds calculated from SEM analysis.Porosity (%) MWCNT:BP by SEM image ratio processing   1:0.5 43.424 ±2.88 1:1 44.121 ± 3.66 1:2 39.895 ± 2.72 1:3 32.389 ± 4.93 1:4 23.623 ±2.02

In addition to microCT and SEM image processing, LEP was performed toassess the porosity of MWCNT scaffolds. LEP is a widely used, IUPACrecommended, non-hazardous (no mercury) method to assess the porosity ofceramics, food products and nonwoven fibrous filter media beds. Theporosity (%) and median pore diameter for all MWCNT scaffolds (MWCNT:BPmass ratios between 1:0.5 and 1:4) is presented in FIG. 6C. The resultsshow a decreasing trend in porosity and average pore diameter as afunction of MWCNT:BP ratio, similar to microCT and SEM image analysis.The macro-porosity and median pore diameter decreased from 94.48% to20.19% and 324.48 μm to 115.87 μm, respectively, with increase inMWCNT:BP ratio. The data represented in FIG. 6C is shown below in Table4:

TABLE 4 Porosity and median pore diameter of MWCNT scaffolds determinedfrom liquid extrusion porosimetry. Porosity (%) by MWCNT:BP liquidextrusion Median pore ratio porosimetry diameter (μm)   1:0.5 94.485324.48 1:1 85.684 312.96 1:2 68.275 288.76 1:3 48.305 141.00 1:4 20.194115.87

These results, in addition to microCT and SEM analysis, indicate that anincrease in the amount of benzoyl peroxide leads to a decrease in thescaffold micro- and nano-porosity.

It is hypothesized that the higher amount of benzoyl peroxide may beleading to an increase in the amount of active sites on the MWCNTsthereby inducing a higher degree of crosslinking, and thereby, alteringthe porosity.

Example 6

In the following example, MWCNTs and benzoyl peroxide were mixed in aratio of 1:4 according to the described methods to form a carbonstructure.

The bulk resistivity of the carbon structure was assessed by afour-probe resistance measurement technique (Signatone S302-4, SP-4probe) at Center for Functional Nanomaterials ((TN), Brookhaven NationalLaboratory, New York. Four point resistance measurements assess planarresistances for a theoretically infinitesimal thickness of sample. Thus,bulk material resistance can be derived from sheet resistance with acorrection factor (F) to account for the thickness of the sample. Thefour, spring-loaded probes were equally spaced at 1.25 mm distances,with the two outer probes providing current and inner probes measuringvoltage. Sheet resistance values for each MWCNT scaffold were measuredat three different regions. Resistivity of the MWCNT scaffold wascalculated by:

$\rho = {R_{sheet}*w*\frac{\pi}{\ln (2)}F}$

where ρ is the bulk resistivity, R_(sheet) is the sheet resistance, w isthe thickness of the sample (0.5 cm), and F is the correction factor.The conductivity was then obtained by calculating the 1/ρ value.

The bulk electrical conductivity of the MWCNT scaffolds (cylinders, 6 mmlength, 5 mm diameter) was calculated to be 2×10⁻¹ Scm⁻¹ from four pointresistivity measurements, satisfying the conductivity requirements for alarge number of electrical applications. This electrical conductivityvalue is similar or higher than a large number of thin films preparedusing carbon nanotubes or graphene with large networks of sp² carbonatoms, and scattered regions of sp³ carbon atoms, but lower than thinfilms of carbon nanotubes or graphene with only sp² carbon networks.

Thus, the Raman and conductivity results taken together imply that thechemical composition of the MWCNT scaffolds mainly comprises of sp²carbon networks with sp³ carbon junctions at the crosslinking sites.

While the present disclosure has been particularly shown and describedwith respect to specific embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details may be made without departing from the spirit and scope ofthe present disclosure. It is therefore intended that the presentdisclosure not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

1-13. (canceled)
 14. A carbon structure having an elastic modulus fromabout 16 MPa to about 84 MPa and a hardness from about 0.7 MPa to about4.9 MPa.
 15. The structure of claim 14, wherein the carbon structure hasa pore diameter of between about 125 nm and about 325 μm.
 16. Thestructure of claim 14, wherein the carbon structure has a porosity ofabout 20% to about 95%.
 17. The structure of claim 14, wherein thecarbon structure has a dimension greater than about 1 mm.
 18. Thestructure of claim 14, wherein the carbon structure has a bulkelectrical conductivity of about 2×10⁻¹ S cm⁻¹.
 19. The structure ofclaim 14, wherein the structure is formed of a carbon material selectedfrom the group consisting of multiwalled carbon nanotubes, single walledcarbon nanotubes and combinations thereof
 20. The carbon structure ofclaim 14, wherein the carbon structure further comprises a radicalinitiator.
 21. The carbon structure of claim 20, wherein the radicalinitiator is selected from the group consisting of a peroxide, benzoylperoxide, a compound with a peroxide functional group (ROOR′), methylethyl ketone peroxide, Di-tert-butyl peroxide, benzoyl peroxide, acetoneperoxide, bisacylphosphine oxide (BAPO), aluminophosphate (MAPO),tert-Amylperoxy-2-ethyl (TAPO) or azobisisobutyronitrile (AIBN) andcombinations thereof.
 22. The carbon structure of claim 20, wherein theratio of carbon material to radical initiator is between 1:0.5 to 1:4.